In recent years, development of liquid crystal display devices (LCD) has increasingly been made aiming at a decrease in weight by decreasing the depth, an increase in the image area, and enhancement in definition. Along with such development, a decrease in thickness, an increase in width, and higher quality have increasingly been demanded for a polarizing plate protective film. Consequently, accompanied with the enhancement of image quality of liquid crystal display devices, the level in quality of optical films has become more severe.
Commonly and widely employed in the polarizing plate protective film is a cellulose ester film or a cycloolefin film. Along with the recent increase in the screen size, demanded has been a wider and longer roll film.
Incidentally, in either a solution casting film production apparatus or in a melt casting film production apparatus, in cases in which the film edges are slit off between the completion of peeling and the winding, problems have occasionally occurred in that peeled substances (slit scrap) from edge slitting portions of the film are generated.
Further, problems have occurred in that, during casting, film is torn, and along-with generation of slit scrap, so-called “edge rise” is formed at the film edges.
Patent documents, which relate to overcoming of the problems occurring in the film edges during a slitting process include the following:
Patent Document 1 discloses a slitting machine having a beveling portion so that it is possible to slit film with high quality, adhesion of film-slitting dust to a rotary blade is minimized and the slitting capability is maintained for an extended period of time.
Patent Document 2 discloses a film slitting method in Which, between the casting step and the winding step in a film production process employing a solution casting film production method, a step is further provided in which edges are continuously slit off along the film conveying direction while the edge dust is removed. Further, in the film slitting method described in above Patent Document 2, the residual volatile amount of the film during slitting is specified.
Patent Document 3 discloses a film slitting method in which, in the same manner as the case of Patent Document 2, between the casting step and the winding step in a film production process employing a solution casting film production method, a process is further provided in which edges are continuously slit off along the film conveying direction while the edge dust is removed. In the film slitting method of above Patent Document 3, the temperature of the film slitting portions is specified to be less than the glass transition temperature (Tg).
Patent Document 4 discloses an edge dust removing method using air flow, in which the edge dust is further cut in the longitudinal direction into thin strips to easily blow the thin strips using an air flow.
Patent Document 5 discloses a film slitting method in which in order to decrease the formation of slitting dust during slitting and to extend the exchange period of the slitting blade by extending its working life, the edge of a disk-shaped slitting blade is inserted, in a non-contact state, into a groove formed on the film support slitting base.
Patent Document 6 discloses a film production apparatus provided with an edge slitting device which slit off film edges while the slit dust is blown by an air flow, wherein the volatile solvent contained in the air employed for blowing the dust is subjected to adsorption removal employing an adsorption recovery device.
However, when the edges of a recent wider optical film are slit off, it was found that uniformity in optical property in the lateral direction of the optical film was not fully obtained, specifically, uniformity in the retardation vale in the lateral direction of the optical film was not fully obtained, only by reducing the slit scrap and edge rise, as described above.
The above patent documents only describe methods to decrease the slit scrap and to minimize the edge rise, and have not fully overcome the above drawbacks of the wider optical films.
An object of the present invention is to solve the above problem of degradation of optical performance in the lateral direction of the roll film which occurs when the width of which is increased to 1400 mm or more, and to provide an optical film exhibiting excellent quality, for example, uniformity in the retardation value in the lateral direction of the roll film, by conducting a quality control by which slit scrap is reduced, tearing of the film is avoided and the film dimension is stabilized.
Patent Document 1: Japanese Patent Publication for Public Inspection (hereinafter referred to as JP-A) No. 2001-315089
Patent document 2: JP-A No. 7-11055
Patent Document 3: JP-A No. 2003-89093
Patent Document 4: JP-A No. 2003-291091
Patent Document 5: JP-A No. 2004-50307
Patent document 6: JP-A No. 2004-66505